Determination of the electromagnetic character of soft dipole modes solely based on quasicontinuous γ spectroscopy

Voinov, A.; Schiller, A.; Guttormsen, M.; Rekstad, J.; Siem, S.

doi:10.1016/s0168-9002(02)01798-9

Nuclear Instruments and Methods in Physics Research Section A:

2003

DOI: 10.1016/s0168-9002(02)01798-9

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Determination of the electromagnetic character of soft dipole modes solely based on quasicontinuous γ spectroscopy

A. Voinov

A. Schiller

M. Guttormsen

et al.

Abstract: We show that the combined analysis of the quasicontinuous γ spectra from the ( 3 He,α) and the (n th , 2γ) reactions gives the possibility to measure the electromagnetic character of soft dipole resonances. Two-step γ-cascade spectra have been calculated, using level densities and radiative strength functions from the ( 3 He,αγ) reaction. The calculations show that the intensity of the twostep cascades depends on the electromagnetic character of the soft dipole resonance under study. The difference reaches 40-… Show more

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Cited by 5 publications

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“…It has been shown that TSC intensities from ordered spectra can be used to investigate the soft RSF [12,13]. The TSC technique for thermal neutron capture has been described in [14].…”

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confidence: 99%

Large Enhancement of Radiative Strength for Soft Transitions in the Quasicontinuum

et al. 2004

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Radiative strength functions (RSFs) for the 56,57 Fe nuclei below the separation energy are obtained from the 57 Fe( 3 He, αγ) 56 Fe and 57 Fe( 3 He, 3 He ′ γ) 57 Fe reactions, respectively. An enhancement of more than a factor of ten over common theoretical models of the soft (Eγ < ∼ 2 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) is observed. Two-step cascade intensities with soft primary transitions from the 56 Fe(n, 2γ) 57 Fe reaction confirm the enhancement.PACS numbers: 25.40. Lw, 25.55.Hp, 25.20.Lj, 27.40.+z Unresolved transitions in the nuclear γ-ray cascade produced in the decay of excited nuclei are best described by statistical concepts: a radiative strength function (RSF) f XL (E γ ) for a transition with multipolarity XL and energy E γ , and a level density ρ(E i , J π i ) for initial states i at energy E i with equal spin and parity J π i yield the mean value of the partial decay width to a given final state f [1]Most information about the RSF has been obtained from photon-absorption experiments in the energy interval 8-20 MeV, i.e., for excitations above the neutron separation energy S n . There, the giant electric dipole resonance (GEDR) is dominant. Data on the soft (E γ < 3-4 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) remain elusive. Corresponding data from discrete transitions show large fluctuations and are biased toward high transition strengths due to experimental thresholds. First data in the statistical regime have been obtained from the 147 Sm(n, γα) 144 Nd reaction [2]. They indicate a moderate enhancement of the soft E1 RSF compared to a Lorentzian extrapolation of the GEDR. For spherical nuclei, in the framework of Fermiliquid theory, this enhancement is explained by a temperature dependence of the GEDR width [3], the Kadmenskiȋ-Markushev-Furman (KMF) model. However, the experimental technique requires the presence of sufficiently large α widths and depends on estimates of both α and total radiative widths in the quasicontinuum below S n . The sequential extraction method developed at the Oslo Cyclotron Laboratory (OCL) [4] has enabled further investigations of the soft RSF by providing unique data for transitions in the quasicontinuum with sufficient averaging. For deformed rare-earth nuclei, it has been shown that the RSF can be described in terms of a KMF GEDR model, a spin-flip giant magnetic dipole resonance (GMDR), and a soft M 1 resonance [5,6]. In this work, we report on the first observation of a strong enhancement of the soft RSF in 56,57 Fe over the sum of the GEDR and GMDR models. This enhancement has been found in Oslo-type experiments and is confirmed independently by two-step cascade (TSC) measurements. To our knowledge, there exists at present no theoretical model which can explain an enhancement of this magnitude.The first experiment, the 57 Fe( 3 He, 3 He ′ γ) 57 Fe and 57 Fe( 3 He,αγ) 56 Fe reactions, was carried out with 45-MeV 3 He ions at the OCL. Particle-γ coincidences were measured ...

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“…It has been shown that TSC intensities from ordered spectra can be used to investigate the soft RSF [12,13]. The TSC technique for thermal neutron capture has been described in [14].…”

mentioning

confidence: 99%

Large Enhancement of Radiative Strength for Soft Transitions in the Quasicontinuum

et al. 2004

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Low-energy M1 excitation mode in 172Yb

et al. 2006

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The low-energy M1γ-strength from radiative proton capture experiment

Voinov

Grimes

Brune

et al. 2015

EPJ Web of Conferences

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Abstract. The multipolarity of the low-energy (< 3 MeV) γ−strength function has been experimentally studied with the 55 Mn(p,2γ) 56 Fe reaction at 1.65 MeV proton beam energy. The spectrum of two-step γ−cascades populating the ground state of 56 Fe has been measured and compared with calculations assuming that E1 or M1 multipolarities dominate in the low-energy region. It was found that experimental data points are reproduced only if the assumption on dominance of the M1 multipolarity is used.

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Determination of the electromagnetic character of soft dipole modes solely based on quasicontinuous γ spectroscopy

Cited by 5 publications

References 20 publications

Large Enhancement of Radiative Strength for Soft Transitions in the Quasicontinuum

Large Enhancement of Radiative Strength for Soft Transitions in the Quasicontinuum

Low-energy M1 excitation mode in 172Yb

The low-energy M1γ-strength from radiative proton capture experiment

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